Catalyst systems for the stereospecific polymerization of olefins are widely known in the art. The most common type of catalyst system belongs to the Ziegler-Natta family and comprises a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound, used in combination with an Al-alkyl compound. Conventionally however, when a higher crystallinity of the polymer is required, also an external donor, usually an alkylalkoxysilane, is needed in order to obtain higher isotacticity. This is because usually, when an external donor is absent, the isotactic index of the resulting polymer is not sufficiently high for many applications even if a 1,3-diether is used as internal donor.
In certain applications in fact, such as in thin wall injection molding (TWIM) production, it is necessary to use polymers endowed with, at the same time, relatively high fluidity i.e., with a relatively lower molecular weight, medium broad molecular weight distribution and high isotacticity in order to have high quality moldings.
Most of the catalysts belonging to the field of heterogeneous Ziegler-Natta catalysts are not able to provide high fluidity propylene polymers at standard chain transfer concentrations but are able to generate propylene polymers with medium/broad MWD and high isotacticity. An example of such catalysts is constituted by WO/2010/078494 which discloses ZN catalysts based on the use of hydrocarbyl substituted phenylene aromatic diesters as internal donors. Such catalyst components can be used together with an external donor selected from alkylalkoxysilanes, carboxylic esters and many other. Alkylalkoxysilanes are the preferred external donors. It is also suggested that C4-C30 aliphatic acid esters may be used as activity limiting agent (ALA) in combination with external donors in order to produce self-extinguishing catalyst compositions having reduced activity at temperature higher than 70° C. Esters of fatty acids like laurates, myristates, sebacates, palmitates stearate, oleates are generically mentioned but never tested.
When employing this type of Ziegler-Natta catalysts high fluidity propylene polymers could be in general obtained by greatly increasing the concentration of the chain transfer agent (molecular weight regulator) in particular hydrogen which is commonly used industrially.
This involves increasing the pressure of the reaction system which in turn would make necessary the use of equipments especially designed to withstand to higher pressure which of course would turn into an increased complexity of the plant operation. Notwithstanding the attempt, polymers having high fluidity are difficult to be obtained by this way because the insufficient hydrogen response of the catalyst prevent to reach this target at the maximum operating pressure of the plant.
Accordingly, the conventional way to obtain the desired high fluidity for propylene polymers having also high isotacticity is that of undergoing the chains of low fluidity isotactic propylene polymers to chemical visbreaking using peroxides as radical generators (rheology modifiers). From the operational point of view, however, this is not the preferred way because the stage of chemical visbreaking adds complexity to the entire process. It would therefore be highly advisable to have the said high fluidity polymers available as reactor grades. Moreover, this chemical degradation also dramatically narrows the molecular weight distribution to a level completely unsuitable for certain applications.
Ziegler-Natta catalysts based on the use of 1,3-diethers as internal donor are characterized by increased hydrogen response with respect to other catalysts. However, when they are used together with an external donor of the alkylalkoxysilane type in order to increase isotacticity the hydrogen response is worsened. Moreover, the polymers obtained also intrinsically show narrow molecular weight distribution.
It is therefore felt the need of a catalyst system showing improved hydrogen response, capability of producing, directly in polymerization, polymers with a lower molecular weight in the presence of small amounts of hydrogen and with a medium to broad molecular weight distribution.